The present invention relates to an apparatus and method for measuring ion concentration of a solution by operating an ion sensitive field effect transistor (ISFET) at multiple drain currents and gate-source voltages which allows for diagnostics and control of isopotential points.
An ISFET is similar to a metal oxide semiconductor field effect transistor (MOSFET), but does not have a conductive gate terminal. Instead, an ion-sensitive membrane is placed over the gate or channel region and is exposed to a sample solution. The remainder of the ISFET device is encapsulated. The lead that would be attached to the gate terminal of a MOSFET is attached to a reference electrode. The reference electrode is separated from the ion-sensitive membrane by the solution. An ISFET is typically fabricated with other devices, such as temperature transducers, on a common semiconductor chip which is referred to as an ISFET "die". The ISFET die is mounted within a larger sensor support structure and encapsulated with an epoxy, leaving only the ion-sensitive membrane at the gate region exposed to the solution. The ion-sensitive membrane modulates the gate charge, and thus the potential difference between the reference electrode and the source, as a function of the ion concentration in the sample solution. The potential difference between the reference electrode and the source is commonly referred to as the "gate-source" voltage. One or more of the operating characteristics of the ISFET are then measured and used to calculate the ion concentration.
ISFETs are commonly operated in a constant current mode at only one current. A single device can be made to simulate more than one device by operating at more than one current. This makes it possible to carry out some diagnostics and to control the pH of isopotential points.